1. Field of the Invention
This invention relates to amplifiers in general and, more particularly, to low distortion amplifiers using distortion cancellation techniques.
2. Description of the Prior Art
In the radio art, it is generally the case that the useful sensitivity of a receiver is determined by the front end of the receiver--the mixer stage and preamplifier, if any. The useful sensitivity of any receiver has an upper and lower bound--defined here as the dynamic range of the receiver. The noise generated by the receiver (internal noise) determines the weakest signal that can be received, typically referred to as the minimum discernible signal or the minimum signal which produces a specified signal-to-noise ratio in the receiver's output signal. Conversely, the overload characteristics of the receiver determines the maximum received signal strength that can be received without a predetermined amount of distortion in the receiver output signal. It is generally considered desirable to have a receiver that has high sensitivity without overloading in high signal strength environments--i.e., a receiver with very wide dynamic range.
To increase the sensitivity of a receiver, a preamplifier is added between the antenna and the mixer portion of the receiver. The preamplifier serves to boost weak input signals to overcome the internal noise of the receiver, allowing the receiver to receive weaker signals than without a preamplifier. However, should a strong signal enter the receiver, non-linearities in the transfer characteristics of the preamplifier will add distortion products to the amplified signal. Further, should some strong, unwanted, signals be present in the passband of the receiver, the desired signal will be corrupted by the unwanted signals due to the distortion of the preamplifier--typically referred to as cross-modulation. A remedy that will reduce the gain of the preamplifier when strong signals are present is adding automatic gain control (AGC, typically derived from within the receiver) to control the gain of the preamplifier. However, AGC may not be desirable in all situations, such as in rapidly changing signal strength environments or if the reduction in gain of the preamplifier is so much that the signal-to-noise ratio of the desired signal is no longer sufficient. Another approach is to use active elements and preamplifier circuit configurations which can tolerate large signals without significant distortion and still provide the desired gain with low noise. For example, field-effect transistors have very good overload characteristics, compared to bipolar transistors, when operating class A either with a common source (emitter) or gate (base) configuration. However, depending on the frequency of interest, the gain and noise characteristics of field-effect transistors may not be as good as bipolar transistors.
Selection of the active devices to be used in preamplifier is especially difficult in ultra- and super-high frequency applications. For example, with cellular telephone systems where the operating frequency is approximately one GHz, silicon field-effect transistors do not have sufficient gain and low noise for use as receiver preamplifiers. Using gallium arsenide field-effect transistors instead of silicon devices will achieve the desired gain and noise as well as good overload characteristics, but suffer from relative high cost and the impracticality of integration with other circuit components, such as a mixer, which are typically formed on a silicon substrate for low cost. But for the relatively poor overload characteristics of bipolar transistors when operated conventionally, the ruggedness, low noise, low cost, high gain, and integratability of bipolar transistors would be a good choice for receiver preamplifiers.
Generally, the foregoing is also true for optical receivers. Preamplifiers boost the signal from an optical detector, typically a PIN diode, prior to further signal processing. While the preamplifier is generally necessary for long-haul systems where the received signal energy is weak, in short-haul systems where the received signal is strong, the preamplifier may cause received signal distortion, making the system unusable. It is generally disadvantageous to have two types of receivers (or manually adjust one type) depending on optical signal level received.